The invention relates to a hydrostatic actuator of the type which has two movable component parts defining a variable gap therebetween and a hydrostatic adjustment device operated by working fluid and being springbiased to urge the two parts into a sealing position.
By a hydrostatic adjustment of the gap width, the gap is to be maintained as small as possible so that the variable hydraulic resistance will assume a very high value. In its practical application, this measure will achieve that only a limited overflow will be generated between the two components that normally sealingly engage each other and move relative to each other at a concomitant small friction, irrespective of the thermally and mechanically caused expansion of those components exerting an influence upon the gap width, and irrespective of wear. Pressing of the parts against each other is effected by way of a pressure-impinged member within a pressure-impinged space. Hydrostatic adjustment of play has found a preferred application within the scope of control of hydraulic motors and hydraulic pumps. In the latter machines, the adjustment devices separate the high-pressure side from the low-pressure side. The mechanically elastic biasing means influence the gap width of the components moving relative to each other during standstill and at the moment of starting up.
Hydrostatic actuators with stationary cylinder blocks of the category as constant displacement initially described are known in the construction of motors as well as of pumps, as constant displacement radial piston types as well as axial piston types. The radial piston types comprise mainly those with internally supported pistons, while the axial piston types comprise in the main machines with swash plates. In-line piston models may also have stationary cylinder blocks.
The state-of-the-art furthermore includes hydrostatic actuators such as fixed-vane actuators and gear-type actuators.
Furthermore, pressure-limiting valves are known with such hydrostatic actuators, provided for the avoiding of prohibited stresses caused by forces, torques or pressures and arranged in such a manner that, for instance, the pressure supplied to a motor cannot exceed a certain predetermined value. This measure will reliably preclude damage to components of hydrostatic actuators and/or of the mechanical drive usually arranged subsequent to these, even under extreme conditions, for instance in case of a sudden blocking.
Irrespective of these advantages of hydrostatic actuators versus other drive systems, the known actuators of this category still show a number of disadvantages. Several additional pressure-limiting valves are usually arranged outside of the actuator housing and will act only then, when the actuators are operated beyond the normal load range. Such pressure-limiting valves arranged outside of the actuator housing will then become voluminous and costly, especially with larger nominal cross sections. And it is just with larger nominal cross sections that a considerable expenditure in respect of manufacture and design will be necessary to connect the pressure-limiting valves with the hydrostatic line systems by means of piping and/or hoses as well as threaded joints. In this context, it is certainly possible that with actuators of working-medium flows of about 80 dm.sup.3 /min up to approximately 1000 dm.sup.3 /min, the expenditure for the connection of the pressure-limiting valves will be larger than the expenditure for the valves themselves.